when you go diving like with a gas tank can you breathe in 100% O2 or is that not good? I think 33 feet of seawater is equivalent to 2.2ATA used in HBOT or something

Mostly it's 100% compressed air, it won't be 100% O2

100% O2 would kill you on a regular dive - it is only used near the surface for decompression. Diving on "regular air" is just that, regular air. You can have enriched air which is called Nitrox, a blend of "regular air" and O2. It is usually blended at 32% O2 and sometimes 36%. It affords you longer bottom time as you do not build up as much nitrogen in your blood, however it limits your depth.

There are also "Tri-Mix" blends that technical divers use (special course to exceed 130 ft) and divers who work at extreme depths that remain in a hyperbaric chamber for extended periods of time on the surface (one to two weeks at a time). They breathe a combination of gases.

More can be found here https://en.wikipedia.org/wiki/Breathing_gas / and https://en.wikipedia.org/wiki/Saturation_diving

With pure O2 you are very limited in deep, else the partial pressure of the O2 gets too high. So it is of no practical use beside deco stops near the surface.

Or in case of accidents: first treatment is often O2

So you always use mixes. For the normal diver O2 with N2 (air or Nitrox).

Pure O2 at high pressure is a nasty gas.

100% O2 would kill you on a regular dive - it is only used near the surface for decompression. Diving on "regular air" is just that, regular air. You can have enriched air which is called Nitrox, a blend of "regular air" and O2. It is usually blended at 32% O2 and sometimes 36%. It affords you longer bottom time as you do not build up as much nitrogen in your blood, however it limits your depth.

There are also "Tri-Mix" blends that technical divers use (special course to exceed 130 ft) and divers who work at extreme depths that remain in a hyperbaric chamber for extended periods of time on the surface (one to two weeks at a time). They breathe a combination of gases.

More can be found here https://en.wikipedia.org/wiki/Breathing_gas / and https://en.wikipedia.org/wiki/Saturation_diving

I think Hans Hass had thousands of dives with pure O2....but I guess not very deep and he was lucky...

It is generally accepted to use Oxygen for decompression up to a depth of around 6 meters.

And yes, normally the air in a tank is just that, normal air, with moisture filtered out of the air though. So it feels dry to breathe.

First thing we need to understand is why do we use enriched air for for diving? simply we are using oxygen to reduce the amount of nitrogen in the air mix, this reduces the amount of nitrogen absorbed by the body allowing us to dive for longer.

Why oxygen, probably because it is the cheapest gas around that we can use that is not going to kill us if breathed in any concentration.

Using Oxygen is not without its dangers, in high concentration it is toxic and can kill divers, this is not a problem at normal sea level, but when we dive we breath oxygen at a rate proportional to the mix, times equivalent air pressure, PPO2 (partial pressure of oxygen) that exceeds 1.6 PPO2 we are at great danger. https://en.wikipedia.org/wiki/Oxygen_toxicity

For trained "Enriched Air" divers we can use enriched air often referred to Nitrox if we so wish and it is available, PADI produce two tables EAN32 & EAN36 (Enrcheched Air Nitrox), Nitrox is a mix of Nitrogen and Oxygen, normally filtered normal air with O2 added, but better still if you know where you will be diving calculate the mix you want for for you dive, most dive centres will supply a mix to your requirements, (or there about's), so say you are diving a wreak max depth 30m, for a PPO2 of 1.4, best mix would be 35% and that would give you a max bottom time of 36 minutes, if you use a mix of 36% you can only dive to 28m, for a mix of 32% bottom time would only be 28 minutes, or if you were using conventional un-enriched air at 21% bottom tie would only be 19 minutes.

In actual fact normal air can be toxic at a depth greater than 60m, then you have to use Trimix (or Heliox) but that,s another story, and if you are using high concentrations of Oxygen (>40%) you need your dive gear serviced with O2 rated parts, "O"rings and lubricants, bottles of 100% oxygen are normally hung off the side of dive boats no deeper than 5m for the final stop for divers returning from very deep dives or first aid treatment on board a boat for divers suspected of suffering the bends/DCI/DCS or any other illness related to diving.

First thing we need to understand is why do we use enriched air for for diving? simply we are using oxygen to reduce the amount of nitrogen in the air mix, this reduces the amount of nitrogen absorbed by the body allowing us to dive for longer.

Why oxygen, probably because it is the cheapest gas around that we can use that is not going to kill us if breathed in any concentration.

Using Oxygen is not without its dangers, in high concentration it is toxic and can kill divers, this is not a problem at normal sea level, but when we dive we breath oxygen at a rate proportional to the mix, times equivalent air pressure, PPO2 (partial pressure of oxygen) that exceeds 1.6 PPO2 we are at great danger. https://en.wikipedia.org/wiki/Oxygen_toxicity

For trained "Enriched Air" divers we can use enriched air often referred to Nitrox if we so wish and it is available, PADI produce two tables EAN32 & EAN36 (Enrcheched Air Nitrox), Nitrox is a mix of Nitrogen and Oxygen, normally filtered normal air with O2 added, but better still if you know where you will be diving calculate the mix you want for for you dive, most dive centres will supply a mix to your requirements, (or there about's), so say you are diving a wreak max depth 30m, for a PPO2 of 1.4, best mix would be 35% and that would give you a max bottom time of 36 minutes, if you use a mix of 36% you can only dive to 28m, for a mix of 32% bottom time would only be 28 minutes, or if you were using conventional un-enriched air at 21% bottom tie would only be 19 minutes.

In actual fact normal air can be toxic at a depth greater than 60m, then you have to use Trimix (or Heliox) but that,s another story, and if you are using high concentrations of Oxygen (>40%) you need your dive gear serviced with O2 rated parts, "O"rings and lubricants, bottles of 100% oxygen are normally hung off the side of dive boats no deeper than 5m for the final stop for divers returning from very deep dives or first aid treatment on board a boat for divers suspected of suffering the bends/DCI/DCS or any other illness related to diving.

Thanks that is a great overview! Makes things easy to understand.

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diving gas composition

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